Angle modulated carrier receiver



W. L. CARLSQN ANGLE -MODULATED CARRIER RECEIVER .u \\W v uNNN f @www uwm -i w m WN SQ A u.

Filed May 5, 1944 ATTORNEY www? w. L. CAR!SN ANGLE MQDULATED CARRIERRECEIVER Sheds-Sheet 2 Filed. May 3, A 1944'.

5.0 5.05 5.( Fffaz/f/vc Y- m.. ,lo n 8 4 o 4 8 m m vN. m R f 2, Naz ma-w ATTORNEY ec, 28, i948.

w. L. CARLSON ANGLE MODULATED CARRIER RECEIVER 4 Sheets-Shee't 3 FiledMay s, 1944 INVENTOR Y E. N R m A Dec.- 2%, E948. w. L. CARLSON ANGLEMODULATED CARRIER RECEIVER ,4 Sheets-Sheet Filea May s, 1944 N. WLW

Patented Dec. 28, 1948 ji `UNITED STAT ANGE MODULATED CARRIER inmnviinIn Wendell L; Carlson,v Princeton, 1N. .'J.,.r asisignor toi I f v Radio'Corporation Delaware.

offimerca, agcorporation-of y Application May'S, 1944, Serial No.533,917-

`rMy present invention relates generally to -iInn `proved receivers ofangle" modulated carrier waves, land more particularly to balanced'frequency modulation (FM) receivers which are 'substantiallynon-responsive to amplitude modulation (AM) effects over a wide range oftuning.

The prior conventional FM receiver requires accurate tuning t-o adesired FM carrier 'frequency to `balance out amplitudevariations'vontlie carrier. Under this balancev condition' the .yopposeddemodulator rectiers, usually diodes, carry-` equal average signalcurrent amplitudes. As the receiver is detuned from `resonance with thecarrier or mean frequency of the desired FM Wave, the signal currentsapplied to the respective demodulator-rectiiers may bemadeunequal by thevarying resp-onse of the tuned circuitsof thereceiver. Hence, AM effectsoriginally present on the FM carrier wave or'resulting from detuning ofthe receiver may pass on to the'audio frequency amplifier circuits. One'of the reasons for employing an amplitude limiter prior to thediscriminator section (or FM translating net- Work) of the demodulator,isto reduce undesired AM effects on the carrier wave and thereby avoidthe necessity for critical tuning to the exact center or carrierfrequency of a desired FM wave.

One of the main objects of my present invention is to reduce the needfor critical tuning of an FM receiver, or more generically an ,anglemodulated carrier wave rec-eiver. v

Another important object of my invention is to provide anautomaticcontrol circuit in `an FM receiver, which automatically locksthe center balance (i. e. cross-over), lfrequency of the dis-`criminator network substantially in step with the lmean frequency ofapplied FM Wave energy.

Another important object of my invention is to provide variousarrangements for automatically adjusting, in responseto an undesiredmean frequency shift of an FM Wave relative to apredetermined reference'frequency, the frequency responses of a pair of independent signalltransmission channels feeding a pair of opposedv rectiiers, and theadjustment being made in a sense to accommodate the center balancefrequency of the channels to the mean frequency ofthe FM Wave.

Still other features-Will'best be understood byreference to thefollowing description, taken in connection with the drawing,'in which Ihave indicated diagrammatically several circuit organizations whereby myinvention may becarried into effect.. f f

2s claims. (01.256427) In the drawing: i Fig.fl-'shows.a preferred formof the invention embodied inanFM receiver, c i z Fig.` 2 illustratesideal relations which are desirable-.betweentypical characteristics of.the discriminator and preceding circuits, *.Figs. 3, .-.4 and 5 vshowrespectively differen operational 4characteristics of (the system,`.Figp illustrates 'a modification of the-'invention,l` -V f v Fig.-7`shows another modification ofthe invention. f Y

Referring now to the'accompanying drawings,

`wherein like reference characters inthe different figures denotesimilar circuit elements, there is shown in Fig. 1an FMreceiver ofthesuperletero'd-'yne type.` While my invention is readily incorporated -inany form of receiver of FM Waves,`-Iprefer`to explain the preferredlform of -thelinvention in connection with a superheterodyne receiversystem. It is to be understood that the present invention is notrestricted to reception of'FM waves, since phase modulated l(FM)`carrier waves could be received as well. In general,v I employ Ithe`generic Yterm vangle modulated;wave;in this specicationto include `anFM wave or a PMWave. An FM wave is procomponents'resembling one or'bothof them and is', therefore, a hybrid modulation.

Inthe present application it is` assumed, by wayf'of specific example,"that the receiver is designed tc voperate in the FM broadcast band of4Z-'50 Amegacycles (Md), and that eachtransmitter 'radiates an FM w'avehaving a maximum -frequency range up to l'l5 kilocycles ,(Kc.) with'respect to the normal transmitter frequency.

These arethe assignedfrequency values of-lthe `present-day FM broadcastband. `The receiver may include any desired form of signal collector,

asv for example a dipole l. The collected FMl signal Waves are applied`tothe converter 2 for reduction ofthe vmean frequency value Withoutpreceded by one or more stages of selective high frequencyamplification. The numeral 3 designates the usual tunable selectorcircuit preceding the converter. The circuit 3 is to be understood asbeing symbolic of one or more similar selector circuits whose selectingdevice 4, usually a. variable condenser or adjustable inductor, isadjusted to receive' va zdesiredfllM st'aticni "The device would, ofcourse; preferably be adjusted faccu'- rately to resonate the selectorcircuit to the center or mean frequency of the desired F'NIE station. JY

The numeral generally designates'the tunable local oscillator tankcircuit;` "Tlreireautancedevioe E, usually a variablecondenser-for-adliistahleiine ductor, is customarily adjustedconcurrently with the device 4 so that circuit 5r=willlzbe ituned.-.toa. local oscillation frequency diifering from the de-l sired carrierfrequency by the operatingcinter--- mediate frequency (I. El) Theselective circuits of and preceding converter 2 mamfocoursegbeofthexedl'y tunedrtypepifidesirablei.. 'Eheiritermediate frequency is.usuallychosengfizonnainarnge cf32s to 15;?Mc.sampnrerelyrhyewastotfjexample, 5 Mc. .-The;dotted:lirre;1denotesanyssuitasblelactuiating mechanismzf'or: operating: tlzxef;iselectingdei- ;vices 4:.fand6'... f 'Iheconverter may usetheiwellf knownpentagrid tube;.forr it.; mayouseffseparate oscillator and mixentubesa-Thesevaricusmircuits and circuit components are very well-knownixtothose; skilled: in the-art ot :radio communication, and need only bebriefly referred to.

`The -I. amplificati mayJ embody one cit-more amplifiers selectivelyftunedftothebperating I. 1E. value of. 5. mc.. 0f.A course: all:signalctr-ansmission 4circuitsbetweentthe#collector I- and? thedernoduilator will hefso-.constructedfas tofpassr emciently abendatleast 150 kcsvvide.` It-is alsousual to Adesign the signalVtrransnnission.'1 circuits itc: have a pass band-vofeapproximatelyV200.lrc.-in. width to. providel for.` reasonable tolerances.

The output transformer 9 of the. iinal I. 'F. `amfpliier7 tubehas1itsprimary-fand secondaryr circuits I0 and II. eachtuned tothe I.`Rvalue.. Thepass bandA will.. preferably, asvf'stated-g above; ,.be.chosen toA be. of the. order' of." 200. kc; widevwhile.;the1 mean or.center frequency of Athe-,band is ,5.mc.. `In.l3ig..2

,the curve? anideallrepnesentation. cfs` the.y I. E'. pass, bandcurveofL transformer 9,.it.being under.- stood, of course, that=theprior. I..F..selector circuit-s. will. have similarl passbandcharacteristics;

Inaccordance-withmyinventien-the highy al,- .ternating potentialsidebfcircuit. I Lis; connected in paralleltothe respective .control grids iI2 and I3,.of.` a-pairof amplifier tubes C..and..D., These tubes. by Wayof specific illustration,.may be screen grid.. tubes of the 1852 type;fmylinvention is to be understood,A however, as being. completely independent of' any particular, `tube types.. VThe cathodes of tubes.CfandD are connected'in ccm- :60

mon to the ungrounded.endlflbiasing. resistor I 4,.V thejlatter being.bypassed Ifori' II'SF.; .currents by condenserili. Gri'd 'l2 ofltubei'sconnected to the groundedA end,` ofA biasJ resistor If4 through"aresistor I 6.

The separate'condensers. I T` and- I8 couple' the vrespective .parallelcontrol grids f2' and' I3 "to" the high potential A'side .of i'iigzut'``circuit I I while' the low potential siderof tlreglatteris"gro\n'idef'di The gridL I-3-offtubeiD returnsfordirect currentpotenntial to the grounded -endo bias `'resistor I4 through'a#seriespaetlr'comprisingresistorJ |92,` lead 2112 resistorlzlf,resistorfM.v andi-resistor-Z tofground. The' lowerfend'cf; resistorrlsdsibypassedto fground forr'allc-alternatmg cum:ents;by;corucuscr"r'lThe plates 25 and 26 of tubes C and D are connected to respective tunedcircuits A and B. The common junction of tuned circuits A and B isconnected to a point of desired positive voltage of a direct currentenergizing source (not shown). The Iscreen grids of tubes C and D may,also, be connected to suitable positive potential points ofsaitlifcurrent'asource. Plate: 25is connected, as Yshownftothe-r upperend` of? circuit'y A, the latter comprising parallel-connected coil 21and con- .denser 28. The parallel resonant circuit A is .preferablyadjusted, as by varying the eifective V'inductive'nragnitudie of coil21, to tune the circuit itolmc.,

iCircuitJsB. consists of parallel-connected coil 29 and condenser' 3D",and, as shown, the lower end of the circuitds connected to plate 26. Theinductance of"coi1"29 may be adjusted so as to eesonatericircuit B to afrequency of 5.125 mc. It it, therefore, seen that circuits A and B arenor- .mallysresonateditofrequenciesflocated.onopposite sidessof thecenter. frequency of 5-mc. of-.bandpass .circuits Iii- H Further, the,predeterminedsnor- `rnal frequenciesv of. circuits A andsB'differbyequal g5 amcuntsfromffthe: center. frequency; `The fre,-

quency differencebetweenthe spaced frequencies is 250,kc.,which.iswelkinlexcessofltheunaximum frequency swing. thatwvilly normallybe. encoun- `teredin the=receivedFMsignals.. Circuits A-.and

,3g Bfunction-as the discriminatorV section. ofthe demodulator ordetector network. TheV discrimi.-

na-tor section. functions to develop amplitude modulated (AM) wave.energy from. the. FM` Wave energy delivered to. thev parallel'amplifiers. C `3:, andD;

The AM wave energy derived' from the FM waves isrectied bua .pairofopposeddicdes'Efand G. V'Ilie diode electrodes-maybe in separateA tubeenvelopes, or they may be house in acommcn 4.0 envelope as in the caseofthe Well-known GHS tube. Diode F'l'ias its. anode 3V coupled' to plate25.'. by direct current blocking condenser"32,while its cathode` 33 isconnected'to. the upper en'dLJ of lbedresistor `22'. vThe direct currentreturn re- 'sister 3`4` connects anode 3i to thelower end Hofresistor`22; Diode G'hasits anode. 35' coupled toplate y2I5`lcy`directcurrent blocking condenser 381 Cathode 36 ofthe. diode is connectedtothe grounded end of load' resistor 23, while direct currentl return.resistor 31v is connected' between vthe unground'edend of resistor 23and anode 35. The junction of resistors 34 and 31 'is' directly.connected .to the; junction H off' loadV resistors'v 22 and: 23. Each'of'load' resistors "22 and 123 isshunt'- ediby a: respective one ofcondensers 39 and.' 4U

to'bypass'all I. F: currents;

'The cathode end:.J ofV resistor 22 connectsA to any succeeding audiofrequency ampli'er f'ol- 'loWed' by a suitable reproducer. The point Jis also connected by.-.lead.2i1,A which includes the series.resistorsl2I and-i9,tothe-gridcontrol I3 of tube yD. -The connection-20is'designated asthefBal- .ancef Control?? the. reasons for this;designation will-.homere fullyexplainedzata-.later point.. Theresistorsv 2 I and I9 I and condenser 24'jfunctionto :provide afiltertfori al1 alternating current. corri'- :ponents :.sothat: solelyavariable direct.: current voltage Will be applied to grid I3. Thevoltage `70 vappliectoverr.line 20 variesiin; polarity as well asmagnitude; andL isfderived: similarly f to thev auto:-'maticifrequencyfcontroli (-AFC); voltagewhich' has been# employed: irrpreviously 'known superheterody-nesreceverss tozcorrect. the. tuningefe. thevlocal 17.5 oscillator. However, my inventiomembodiesim.-

5 portant and valuable distinctions, as will hereinafter appear.

Automatic volume control (AVC) bias for the various tubes precedingtransformer 9 may be provided by the AVC connection 4I from the junctionpoint H of load resistors 22 and 23. Filter resistors 42 and condenser43 are provided in the AVC connection in the well-known manner. Thepotential of point H is derived by rectification of the signal impressedon `diode G, and varies in response to variations in amplitude of theapplied carrier wave energy. The function of AVC line 4I is to vary thegain of each controlled tube in a sense such as to maintain the carrieramplitude at transformer 9 substantially uniform despite substantialsignal amplitude variations at collector I. The filter network S2-43Vhas its time constant so chosen as to permit the AVC to respond to slowvariations incarrier amplitude.

Those skilled in the art of FM communication *L are fully aware of thefunctioning of a discriminator-rectifier of the general type disclosedin Fig. 1. A pair of opposed rectiers provided with separate resonantinput circuits, individually detuned in opposite senses by equalfrequency amounts relative to a predetermined center frequency, comprisea system for demodulating FM waves shown by F. Conrad in U. S. Patent2,057,- 640, granted October 13, 1936. In accordance with my inventionthe general Conrad type of i.

'FM demodulator is highly improved, and is provided with means forautomatically-adjusting the center balance frequency of thediscriminator under the control of the signal being received.

Before describing the functions performed by i' my invention, the idealrelations depicted in Fig. 2 will rst be explained. Curve P shows atypical I. Il. band pass curve of network Ill-II feeding the parallelamplifiers C and D. The curve S is a typical discriminatorcharacteristic having i spaced peaks located at the resonant frequenciesof 4.875 mc. and 5.125 mc. of tuned circuits A and B respectively. Theideally linear section between the peaks has its center frequency of 5Mc. located at the center frequency of band pass curve P. When theresonance curve P of circuit I-II (as well as prior selector circuits)is related to discriminator characteristic S of the discriminatorcircuit A--B as shown, the two circuit-s are said to be in perfectalignment. While 3 such alignment is highly advantageous, it is equallyadvantageous to have the mean frequency of the I. F. energy applied totransformer 9 fall at the center frequency of 5 Mc. The dotted verticalline K represents the I. F. carrier or mean frequency. When thecondition shown in Fig. 2 exists, there will be no need for an amplitudelimiter between I. F. amplifier 3 and the discriminator circuits. Solong as noisepotentials in the receiving system do not exceed thesignal-s reception of FM signals will be satisfactory, and it will berelatively free of noise without an amplitude limiter being employed. tShould now the linel K, representing the carrier frequency, be shiftedfrom the centerfrequency position, while remaining within the band P,there may occur .considerable noise ,production or signal distortion,because of amplitude variations resulting from working down `on one ofthe slopes or legs of the band pass response curve. In the past -anamplitude limiter was employed to reduce such eifects, since it wasknown that the ideal relations of Fig. 2 were difficult to secure. Itmay, also, be pointed out that even if the I. F. carrier K is located atthe center of curve 6. P, but the curve S is shifted to oneside'witl'iin curve P. i. e. the tuning of the discriminator circuits isnot accurately aligned with the I. F. selector circuits, unsatisfactoryand noisy reception may result, including beat note interference from anadjacent channel station. Here, again, an amplitude limiter has beenpreviously considered to be at least a partial solution of the problemof reducing the noise. I secure the operating advantages of the perfectalignment depicted in Fig. 2 by providing means for automaticallylocking the center balance (i. e. crossover) frequency of thediscriminator (center of curve S) substantially in step with the appliedI. F. carrier or means frequency (line K). This insures that thediscriminator balance frequency (assumed to be 5 mc.) can be effectivelyaligned with the center frequency of the I. F. amplifier (alsopreferably 5 ma), assuming that the signal is tuned in at the centerfrequency of the I. F. amplifier, even when the discriminator and I. F,amplifier circuits are mistuned due to production tolerances or chang-esin circuit constants during operation and life of the receiver.

The present automatic balance control circuit also reduces thermal andother low amplitude interference for the case when the receiver iscarelessly tuned. It must be understood that curves S and P can bedetermined in the construction of each individual receiver, but that thelocation of K, the I. F. mean frequency, is a variable de-v pendingsolely on how the operator tunes his receiver.

y Considering, now, the manner of operation of the autom-atie balancecontrol system, it should first be noted that the two resonant circuitsA and B are fed from two separate amplifiers C and D, rather than fromone amplifier as is usual in a Conrad type of discriminator.Furthermore, there is preferably no appreciable inductive couplingbetween resonant circuits A and B. Hence, these two circuits tuneindependently of each other. The amplifiers C and D have differentbiasing means, although they have biasing resistor I4 in common. Thevoltage across resistor I4 provides the sole bias for control grid I`2.The grid I3 of tube D, however, receives its bias in part from thevoltage developed across resistor I4, and in part from the directcurrent voltage developed across load resistors 22 and 23 and taken olfat point J. The voltage at point J is, of course, the differentialvoltage of the opposed rectified voltages developed across the two loadresistors 22 and 23. Hence, it is a variable quantity, and causes thegain of tube D to vary in a sense, and to an extent, dependentrespectively upon the polarity and magnitude of the potential of pointJ. The AVC voltage at point H, as stated above, is employed to controlthe gain of preceding tubes, if they are not to act as limiters whenreceiving strong signals.

Considering now the resonance curves, or response characteristics, ofcircuits A and B, the solid line curves A and B of Fig. 3 represent .therespective response characteristics ofthe discriminator tuned circuits.The cross-over or balance frequency of the curves is designated by Fc.This point falls at 5 Mc., and the discriminator is, therefore, alignedwith the band pass network I--I I. Furthermore, let it be assumed thatthe I. F. energy applied to circuit It has a mean or aannam?each'ofzcircuits AzandJBt These-.variable amplitudercurrentsarefrectiedby rectiers FvandiG@ 'Iheirectied currents-.nearing :throughrespective loaclresistors f2-2f and; zzprovidef rectified voltagesofeopposedrpolarity. Pointe will havefa potential which is thediierential of-the'rectied voltages. Hence,A point J willfsupplymodulationevoltage, sincathe polarity and.- magnitude off thathpointdependfonthe sensezand. extenty of.' frequency deviatiomof@ thefsignalenergy-at the. output offamplieiir.:

The mean frequency:of..the-I..F energy, however; remainsat 51Mc. byassumption; and; hence, the?. steady ordirect current voltage: of. pointJ will; remain; substantially constant. The filter Z i--i 4etirkprevents.I anyf pulsating.' or ymo dulation componentsfat J frombein'gf:applied overlinef20. Asllong: as fthe' mean frequency of i the.- I. F.:energy is.5-Mc., andiexceptfor'modulation:frequency potentials which-areremoved. bylter 2l-l-24'-i9f, there. will be-developed. rectified.voltagesv ofthe san-ie magnitude across resistorsaand 23.Y Therefore.theeffectiveldilerential resultant voltage impressedl on grid L3 f is vzerorelative: to f ground. The-:grids '|2.and: I-Sfof amplifiers CandfDWill, themhave'thesamexnegative-bias (i. e., the bias developedacrcsswbiasresistor |41). This-- mea-ns thattubesC `and Dare operatingat the same gain thereby passingzequal.magnitudes-of I-. F. signalenergyfrom circuit H to circuits-A and B.

Y. .Assumepnow-,that for somereasorr the mean Ormarrier frequencyof'theI. F. energy shifts to 5.05 Mc. This could happen because of poor-adj-ustm'ent of-.tuningmechanism T; or because the discriminator yandI.l F. tuning. circuits-fare misalignedand-.O Mc. is at the oenterfofthe I. li'. bandpassfresponse curve.- 'IT-he-effect of themeanfrequencyffshiitsis-to cause .the directzcurrent voltageof point J i tobecome-negative relative to ground.` This-happensbecauserectifier.G hasappl-iedzto itgreater amount ofthe signal atthe meanfrequenoy of- 5.05Mc., since that value is closente theffrequency of circuit -Bfthantocircuit A.. Thefd-.irect current voltage-across resistor 23 exceedingthat across-resistor `2.2, point-J will be effectively. negative;

The 1peint-J becoin'ing.- negative with "respect to ground, gridwill.be= cause-d` izo-becomeI more negative inpotentiah The grid IZwill-beunaffectedin potential-so far as the changefinpotentialoffpointJ is concerned.- 'Iherefore, the gainortubeD Willfbe decreasedfwith respect to i-ts.-.gain when themeanffrequency of theel F. energy.isvat Mc. Since the' vspace 4cturent, of

tube D flows through thef'bias'Y-resistor I4, a reductionin spacecurrent of tube-:D willcause the voltageacrossresistor M to decrease.This causes the. effective voltage oferid l2rto become4 less neg..ative,..and the gain ofv tube C will,4 therefore, increase..y

Hence, it will be furtherv seen that tubefGWiH provideagreater.amplicationof'l. F. signal energy than amplifierY D. In Fig. 3 the.dottedcurves .AuandBi show illustratively theeiect of the thedottedcurves A1 and B1.

Theresu-lt of this action is to movethe discriminator-balance to1a.'.-lr1ie'her frequency,v ap- Qroaching-,thearrierfrequency at: 5.05;Mc. In

Fig; 3f theenew pointi ofi balance onfrequencyof equal response has beenshifted fromFcftofFfe. In'othert words, there has occurred. an zautomatic adjustment ofv thecenter balance frequency of thediscriminator circuit .substantially to the. I.. F. carrier frequency.Such automatic adjustment effectivelyreduces audio y distortion and.interference, as welLv as noise production,f.whichswou1d have. otherwiseoccurred .had the automatic. bal.- ance control circuit. not beenV used.

Should.- theI.A F. carrier frequency shiftto .a frequencyv-lessthan Mc.,for examp1e.4.95.-Mc., then thereverse ofthe.actionidescribedlabove willtake place. Inl thislcase. point. .If Willassumea positive directcurrent potentialA relative. to ground,.due toy circuit Ay receiving;`a.` greater amount of signalcenergythan1circuit B Rectiii'er F=wilbtherefore, .provide .a greater rectified Voltage :across resistor..22. thereby. causing point. J.l to be'positiverelative-to ground. Inthat caserthe control :grid i3 i oftube D will.l be vbiased .less.negativethereby increasingits gain. This will cause the.biasacrossresistorI [4to increase, andthe gain ofi-.tube Cwilhdecrease.Hence, .the balance frequency willl change inthe reverse ofA the.relationfshownfor. A1 and.B1,.ancl thenew balance point .will be at..4.95-Mc.

It/willbe .observedthatonly thegainto circuits AancLB. is cl'ianged;yThe voltages oithecarrier deliveredA tothe two.. diodes. are tobeequalized. The resonancecurycs Auand B1.in.Fig..3.arefthe sameascurvesfAandBl. Only theamplication ofthe-tubesfeeding thesecircuitsA andB has been changed. CurvesAandAi .represent circuit Aandampli'ertubeC.attwo diiirent bias values. The curves. in Fig. 3.may be taken .asrepresentingmaximum degree( 50 Kc.)y ofshift ofthe crossover'. pointvof. the discriminator characteristics which Vshouldbe .providedion.assuming .that vthe difference between the. peak.frequencies of thediscriminator. circuitsis :250 Kc. This isbecause with thecross-overpoint at-5-05 Mathe slope of curveBiisjust longenough to allow fora`signal deviation of. 75 Kc. on the highirequency side of the` carrier.The. invention hasbeen practised successfully. with frequency.differences between dscriminator circuit peaks up to or inexcess of 400Kor.. in Whichcase the circuits-may be designed for increased shiftsofthe crossoverpoint from-.thedesired meannfrequency. In other Words,the greater the frequency dierence. betweenthe discriminator peakfrequencies, the greater may be the automatically-effected shift of thecross.- over frequency.-

From `another viewpoint it' can be said that my invention contemplatesan FM receiver wherein there isprovided a direct current voltage at theoutputlofla discriminator-rectiier, and the volt.- ageis fed back to theinput of Ithe discriminator in .almanner to cause the discriminator.center balance toffollow the carrier` frequency. as the carrierisshiftedover the I. F. acceptanceband during the tuning of. thereceiver system.. The curi/esofFig. 4 show the. directcurren-t voltageoutputlfrom theY demodulator for. unmodulated carrier. inputstotubesC'andfD of l01.5 volt, and at.v frequencies between.4.'7 and 5.3'Mcl Curve .L shows. the variation. in direct'.v current voltage whentheYbalance qcontroli of this invention .isrv not used, while curve. L1.indicatesthe effect-of. using the presentflinventiom'Ilhecontrolcircuit tends to` atten out-fthe typical characteristic L:showing.; that l.substza.ntial a accommodation of the ldiscriminatorbalanceafrequency to the-I. F. carrier value isifconstantlyprovided.. Ifthe; automatic relation shown in Fig. 2 exists.

como? the pass band. The audio voltage output of the y demodulator isplotted against carrier frequency, and the lcarrier is assumed to` beabout 20% amlplitude modulated with 400 cycles, the input being about0.5 volt I. F. Curve E shows the expected.

high increase in audio output as the I. F. carrier departs from the 5Mc. balance value, the present invention not being employed. Curve E1shows the marked action of the control circuit reducing the audiovoltage output to somewhat above 1 volt (as against a peak of 10 volts)when the I. F. carrier has shifted from the center frequency. The AMeffects are` substantially balanced out by a shiftable balance point.ideal relations of Fig. 2 are approximated to the extent that thediscriminator balance frequency is caused to lock in with, or track, theI. F. carrier'within the flat top portion of curve P of Fig. 2.

The following list of circuit -constants have' been used in oneembodiment of my invention as shown in Fig. 1. It should be clearlyunderstood that the following magnitudes are purely illustrative, andthat they are in no way restrictive:

C:56 micromicrofarads (mmf.) C11'= 100 mmf. C18', C17: 80 mmf.

C43 024:0.05 microfarad (mf.)

C: 5600 mmf. C28, C30: 100 mmf.

C39, Cin, C32, 038:33 mmf.

R14: 2.7 megohms R34, Rav: 34,000 ohms R22, R23: 120,000 Ohms R21, R42:1 megohm In the receiver shown in Fig 1 the gain of I. F.

amplifier 8 may be varied either by AVC bias, a

limiter or manually. Actual listening tests with a system of the typeshown in Fig. 1 revealed that thermal fluctuations and otherinterferences, not exceeding the carrier amplitude in intensity, weresubstantially reduced during the operation of tuning the receiver whenemploying the present ybalance control circuit. The noise nullpoint wasbroadened in a manner similar tothe action in a conventional FM receiveremploying a limiter. This action, it will be noted, is predictable from4balance control circuit restored the discriminator ,center balancefrequency to the middle frequency of the I. F. amplifier as the carrierwas tuned to that frequency. A combination of thermal .iiuctuationsignition impulses and beat note interference is most effectively reducedin its interference effect on the desired signal, when the That is, theeffective yreduction takes place when the center frequency ofthe I. F.amplifier and the balance The i0 frequency of the discriminator are bothin tuile with the I. F. carrier frequency. This is also the conditionfor minimum audio amplitude distortion.

In Fig. 6 I have shown a modification of the invention; separate AVCconnections are employed in place of the automatic balance controlcircuit of Fig. 1. Furthermore, each of amplifiers .C and D areconcurrently controlled with a respective one of a purality of cascadedI. F. amplifiers feeding the parallel tubes C and D. Referring to Fig.6,the I. F. amplifiers are designated as 50 and 5| respectively. ,Theseamplifiers may be of any well-known form. It will be understood that theinput transformer 52 `of amplifier 50 will have a band passcharacteristic similar to that represented by curve P in Fig. 2.Similarly each of I. F. transformers -53 and 9 will have a band passcharacteristic as represented by curve P in Fig. 2. Amplifiers C and Dhave their signal control grids I2 and I3 connected in parallel throughthe respective direct current blocking condensers I1 and I8 `to the highpoten'- tial side of the resonant secondary circuit of transformer =9. i

It will be recognized that the .opposed rectifiers F and yG are includedin rectifier circuits connected to the respective discriminator circuitsA and B in the same manner as was described in connection with Fig. 1.The essential difference in the system shown in Fig. 6 from that shownin Fig. 1 is that separate AVC connection lines are.

taken from the respective anodes 3| and 35 of the two diodes. The actionof this system is substantially the same as that of Fig. 1, and thecurves of Fig. 3 apply equally well to Fig. 6.

One of the AVC connections comprises a lead 54 which is connectedtoanode 3| of diode F, and includes the filter network consisting ofseries resistor 55 andi-shunt condenser 56. vThe lter network 55--55suppresses any pulsating voltage components -in the voltage applied overline54 to the grid circuit of I. F.. amplifier 50. It vwill be undertoodthat the AVC connection to the signal-grid circuit of amplifier 50 ismade in the well-known and usual manner to the low potential `side ofthe tuned secondary circuit of transformer 52.4 The AVC line 54 is alsoconnected to the control grid |2 through the resistor 51.

The second AVC connection comprises the line 58 which includesthe seriesfilter resistor 59 and shunt condenser 60. yThe function of filternetwork 59--60 is, also, to suppress any pulsating voltage component inthe voltage transmitted over line 58 to the grid-,circuit of amplifier5|.

vHere the AVC connection is made to the low potential side of theresonant secondarycircuit of I. F. transformer 53.. The resistor 6IYconnects the AVC line 58 to the control grid |3-of amplifier tube D. Itwill, therefore, be seen that AVC connection 54 applies AVC bias to thesignal control grids of I. F. 4amplifier 50 and amplifier C, whereas theAVC connection 58 controlsthe signal grid biases of I. F. amplifier 5|and amplifier D. The AVC connections vare the sole sources of variablebias in this modication. l,

In actual loperation the AVC voltage which is applied over connection`54 to amplifiers 50 and C, is obtained from the rectified volta-gedeveloped across series-arranged resistors 34 and 23. On the other handthe AVC voltage for amplifiers 5| and D is obtained from the rectifiedvoltage developed across series-arranged resistors 3'|.and 23. l

encre/2o?? control of Ldirect current voltagefacross Tesis-tor 23 gWhicliw aiects -bctn channel-s. =In general, r"the vseparate fAVClconnection/sfeilliani ifunctionito control the transmission of signalenergyftorthe -discriminatcncircuits A anfPB "inV-mfuchthe same fashionas Was'explai-nedfin connection with `-Figs. 1'-tojff'inclusive. 'As-the-'cei;1".eierL frequency overlthe 1frequencyffband;'thegainioff tubes 1Cand iD respectively risesanil falls,`| or'AViceversar-so asautomatically L-to hold the voltageto L'the -1 diodes hearlyconstant. lI

-discrimixiator section Aof ethe demodulator hed 'fnot be vof the.Conradftypeshown f inS Fig 1. Attrnazybe of the type ydisclosedand'Iclaimedfby S-W-1=Seeley iff-T LS. Pa-tentlNo-f-2j121g103, lgranted d une412d, i938; tn'thatcase, land ass'hown in presentFigg '7, the pairotfresultant 'vector voltagesliormed lin the 4rSeeleylty'p'e ofidiscriminator arefseparatelyk amplified priori-to :rectication,- and'the )relative amplification may '-be -fcontrolled `in tiles-mannergenerally .described Uin Iconnection with 41.' vInitis-ieflsyfstern l ofFig. ithe 4receiver is .similar Kkto a conventional ''EMvfreeeilverI-upfto tubes GiandID.

The I. F. amplier 8 in the modicationofiig. L'Iifiss'howninlmorefdetafil ithan fin E'ig. f 1.I VlHoW- ever, v4itwvvilltbe-recognized that :ampi-nier` 8 is vconwentional *lnfsconstructioniandrconnections. `-Its finput l transformer $10 i .-a -band `'pass-characoutputatransformer 5H (hasfIthe-midpoint if:itssecondaryiwindingn iconnect'ed 1 directly `to .thefplatefendofprimaryfwiniiingI-S.' {Shuntlcondenser. tunesrwindingllftothefoperating I.`F.

Thefshunt 'condenserv-liftlmes'iwinding 12 fto the sttme-- I. :F.walue`The Q'resonance curve --of imag- .netically 'coupleddcircuitsdile-13land 11-2-15'is fsim'i'la-r tof-thabof .curveiP effing-i2; 'Duetotheconnections. betweenthe4 primaryS-i and Jvsecondary;circuits-fof.:transformer-:H4 t'hjere tiis -rprovided the 'rfreeiuenrnrydiscriminateryaction disclosed iin the Laforesaidiseeleypatent. f

'The oppositeidesrof second-ary ,ir-zireuit"y l2-L15 :are connected Ltoi-the oresp'ectivef signal `control gridsiiI 2'farndaISJofiamplierrtubes10?andDbyrespective. direct' current 1 'ihlooking-f-'condensers l' and'l Ilhe. plates :2251 andfZBeo'f ampliiers-fC andiDareconnected Itoiopposedidesot fthe-res- .',onantaoutputscircuitscandlfIhe i844 *terfminal of `tthe :direct cnrreritlsource lis': connectedinficommon.itoitheiunction otrthe'fcircui-ts 80 and 81.' Resistor,lbypassed for-ZI. Fi currentglprof-Vides ga common l negativenbiasfiorzboth grids I2 `and. '.413; ithisvli-s similar fto: the functioni-ofithe cathode..hiasIr resistor:` in FigL f 'I-'hescircuitsr ian'dli,araeach Itunedito the nperatinglflvailue. 'The :opposed -rdiodes i'Fand G :have f respective :resonant .inputlcircuitsial and 183.ryfIhe--junct-;ion L.of series load resistors 222 v'and 28,?.each of theflatter.- beingabypassed 'lbyi respective conx'ieI-lse-rs L38 .andMorLfiF.;currcntslisonnected directly :to the :junction actresonant:input circuits iandi8'3. TheACIatter-input :cir-cuits` are, LalsopeachAtuned to ltheroperati-ng Ik ivalue: Rosana-nt circuits *8U and 82 aremagneticallyffeouoleitofrprovide' -a fba-ndjaassnetworkvv Whose responselcharacteristic `is a typieavlfbaind pass 'curve gdepi'cte -Jby c u'rves'abovethe 7coupled circuitsBd-BZ 'In the same ical of *the-inagneticallir coupled; resonant circuits putude signalenergytol*respective-remainsfr* tand fG. The magnitude of sign-a1renergy transmitted through these lioand-pass networks :isautomaticallycontrolled -in response tofany shift in the centeror-*mea-n" requ ency` of the I; s'ig-` nal `energy fromlthefpredeterrninedI referencefreiquency ofthe-discriiniriator'circuits;`

f 'The automatic balance 'control circuit f-isrpro-v.vided'byv:cminecting the 4cathode end `of resistor A2.21to A.the`control grid'f-l bynieans -of a 'lead" 92. .TheleadSZfncludesftheseries -lter v`resistors '98 fendi, ithe junctionof these resistorsbeingfby.- `passed toercun-d for `alterna-ting `current's-`l;iycondenser Si; It vwill, therefore, loeA seen that :thelrcontrol `gridIyas inthe 'caseof tFig. 1,'is returned-'to the vgrounded end'of biasresistori 'through :the series-connected load resistorsf'22 .-andzl3.This mea-nsthat'fva-riations impotential `atthetcathode end ofA'resistor'-will causeavarilationiinllaiasoffgrid143 with e concurrentchange :1n..gain:.o'f.,ampli1ier 13. The usual AVC circuit '$961 is:connected from -the junction of resistors22 vanni 52.31110 the signalgridsof those tubes lvs'lhich areitoibe controlled .in y-gain toreduceslow yVaria- ?.tions Lin 4carrieramplitude 'The-AVC circuit isshown connected through lter resistor -`9I ito @the lowafpotentialvsidefof Ithe'ftunedv `input circuit ,of FI; JF. amplier J8.

VThe.operation of the --system of SFig. ff-iis, "in general, similar tothatdescribedin connection :with Ithe :system :of Fig. 1.Thelvfrequencyfdiscriminator which yprovides the-.rresiciectiveresultant vector voltages-tor grids-,l2 and I3 of amplifier tubes C andvYD 1Afunctionsin.. the Wellknown manner described :in .the'feeleypatent. There is induced in the Ituned. secondary circuit of transformer'II a pair .offsignalvoltages- One of these voltages is directlyappledcto 4:thesesondary circuit from the `primarycircuit at themidpoint of the cQlLZ.: .Since grids 1I2 and I3 are connectedto..themidpoint.oifcoili12 in parallel relation, the I. .F. Voltage.will loe-:applied in parallel to control grids I2 and I 3'.y On theother hand, thefmagnetic coupling between the :coil 13 'inthe I. voltageinduced-inthe secondary --circui-t.- Y

ThisLphase-sh-ifted'. l1.'volta1ge'willl be "applied in -push-pullrelation @to 4grids A'122' and I3 relative ito thefmidpointoficoilqlHence, f-each^'g1icll I2 a-ndIS fhas :applied to -ita parallel fI. "F.voltage -cor'nfponent j-ancl-a`J 'phase-shifted'voltage lcom-- lponentgfbut the lattercomponents b'eingof oppovsite polarity. Therefore, thereWillibe eiectively applied 5to-'each rcon-trolgrida yresultant 'vectorvoltage 'These resultant -vector `voltages are of #extent vof theaforesaid frequency shift.

In l'7 3I have-"for -thepurposes of specific illustration only, depictedthe vectors representing lthe lparallel 4voltages and the phase-shiftedvoltages as Welfla'sith'e resultant veetorfvolt-ages.

"These-vectors are Shown at theinput connections --toy tubes C and D.vThe-resultantWoltafges are :shown in solid lines; While `vthequadrature-related v'volta-ges Aare shown in dotted-lines. 'The-'vector"rel-ations 'shown in Fig. 7 `are ffor the casefwhenthemeanl-frequeneyof `the I.'r F.-energy\is equal This will indicatethat the effective magnitude of the.. resultant voltages transmittedthrough the parallel amplifier channels C and D are of equal magnitudeat the balancefrequency of the discriminator network, and that thevariations of each resultant vector voltage occur within the responsecurves 90 and 9|.

It will now be seen that when the mean frequency of the I. F. energyoutput of amplifier 8 shifts relative to the predetermined balance orcenterfrequency of the discriminator network, then one of the resultantvector voltages will exceed the other in magnitude. This will result inthe development of a voltage at the cathode end of cresistor 22 whosepolarity and magnitude will depend upon the direction and extent of thelast mentioned frequency shift. This voltage will vary the bias of gridI3 thereby to change the gain of tube D. The latter change causes thegain of tube C to vary in an opposite sense and, to a degree such as toequalize the effective magnitudes of the resultant vector voltagestransmitted to rectifiers F and G. v

. It will be seen that in the-form of the invention shown in Fig.' 7,there are located parallel amplifier channels between a Seeley type ofdiscriminator and the opposed rectifiers. The gain of ,each amplifierchannel is controlled as in Fig.

l, b ut unlike the latter the frequency discrimination is provided priortothe amplifier tubes C and D. However, the curves of Figs. 3, 4 and 5apply equally well to the system of Fig. '7. In common with the systemsof Figs. 1 and 6, thereis provided in the system of Fig. '7 means forautomatically; controlling the gain of the parallel channels so as tofeed substantially equal carrier voltages tothe rectifiers.

While I have indicated and described several systems for carrying myinvention into -effect,.

it will be apparent to one skilled in the art that my inventionis by nomeans limited vto the particular organizations shown 'and described, butthat many modifications may be made without departing from the scope ofmy invention.

WhatI claim ist, l

1. A frequency discriminator-rectier circuit comprising a signaltransmission channel including a pair of .opposed rectiers each havingan output load impedance, said rectifiers providing an output potentialwhich is the difference between the potentials created across theirrespective load impedances, resonant circuit means coupled to saidrectifiers to provide different respective amplitude-frequency responsecharacteristics` with different peak frequencies,4 said resonant circuitmeans having frequency response curves which cross each othersubstantially at said input cir-cuit frequency, means for 'applyingfrequency modulated wave'energy `to said resonant circuit means, andamplifier means provided in said signal transmission channel forautomatically locking the =cross-over point .of the frequency responsecurves of the discriminator network substantially in step with the meanfrequency of the applied frequency modulated wave energy.

2. A frequency discriminator-rectifer.circuit comprising a pair ofopposed rectifiers each "have `ing; an output yload impedance, said.rectifiers providing an output potential which is the dif--l ferencebetween the potentials created across their respective loadimpedances, asource of frequency modulated carrier energy, resonant .circuit meanscoupled to the inputs tosaid rectifers to provide different respectiveamplitude-frequency response characteristics with different peakfrequencies,*said resonant circuit means having frequency `responsecurves which cross each other substantially at said input vcircuitfrequency, and means including an amplifier and coupled -to saiddiscriminator-rectier circuit for automatically changing saidcross-overA point of the frequency response curves, as viewed from saidsource of frequency modulated carrier energy, upon change in the carrierfrequency of the received signal energy.

43. A frequency discriminator-rectier circuit comprising a pair ofopposed rectiers each having anl output load impedance, said rectifiersproviding an output potential which is `the difference between thevpotentials created across theirvrespective load impedances, a selectivecir. cuit substantially symmetrically responsive to frequencies adjacentto a center frequency corresponding to a preferred v carrier` frequency,resonant circuit means coupled to said rectilers to provide differentrespective vamplitude frequency response characteristics with differentpeak frequencies, said resonant circuit means having frequency responsecurves which cross each other substantially at said input circuitfrequency, and amplifier means coupled vto said recti-ers forautomatically changing saidcrossover point of the frequency responsecurves,` as viewed from said selective circuit, upon change inthecarrier frequency of the received signal energy and in the' samefrequency direction as the change in the received carrier, frequency.v

4..In' combination, a pair of opposed rectifiers each having an outputload impedance, said rectiers providingan output potential which is thedifferenceV between the potentials created across their respective loadimpedances, a source of frequencyv modulated carrier energy, circuitmeans for feeding energy from said source to said'rectiers including adifferent channel for each rectifier, and an amplifier in each of saidchannels responsive 'to the rectifier output potential. for reducing theefficiency of transfer of energy to one of said rectifiers through itsrespective channel upon change of the carrier frequency in onedirection.

5. In combination, a pair of opposed rectifiers each having anv` outputload impedance, said rectifiers providing an output potential which isthe difference between the potentials created across their respectiveload impedances, a vsource of frequency modulated carrier energy,circuit means for feeding energy from said source to said rectifiersincluding an individual channel for each of said rectiers, and.amplifier means in said channels responsive to the rectifier outputpotential for decreasing the efficiency of transfer of energy to one ofsaid rectifiers through its associated channel and simultaneouslyincreasing the efciency of transfer of energy to said other rectifierthrough the other of said channels, or vice versa, depending upon changeof the carrier frequency in one direction to the other respectively.

6. Afrequency discriminator-dete-ctor circuit comprising a pair ofopposed'rec-tifierseachhlaving an output load impedance, said rectiersprovidingan output potential :which is the differencebetweeftheptentiaisereatedwaerss .theresespectivetoad `impedances', a-1'source Afoffrequency modulated-carrier energie -a-r circuitYforifeedirrg energy from* saidtsom'ce-V tesa-idrcotiiersfthrough a"'different channelffor each-i ofesaid rectifiers, and v'acmimy -tubeamplifier fcircuitsl in said-*ehemals ooupiedito -said output/loamimpedances andre.- sponsive` tof the 'reetiiier output `'potential ffordecreasingfthe eiic'zienoyofi-transferof:energyato one of 'fsaidreetiiers-f throughits :associated channel and simultaneously increasingftheeiiiciencyfof transfer off-energyfto =said f other rectifiertlnfough the other ofssaid channels; f "7; iIn/combinatiomJa paircof`opposed-"reetiiens eaehihavingrfanf output loadimpedan-oe; saidrectiers providing aan output ipotentiai .which iis :the differencebetween the'` potentials createdf across their respeotiveioad:impedanoesm soureeofrangle modulated carrier energy, lmeans :'for cedingenergy-ffromsaid2 Sourcen toA saidfrectifiers through different channelsrespectivelmand meansrin said channels.,v responsive .'.todirect'current potentials acrosslfsaid. i impedanccs for .decreasing "the.efciencia-:oftransfert of.' energy .to ionesofssaiiil :rectiers throughf its respective channel :reiatively to thefeffieieney.' of transfer:cf= energy .2to:.said.other reetier 1 through fthe i other "or saidchannels, 1er vice versa,i.depending1upon change, of thef carrierIr'equency'iin one .direction s or :the :other 'respectiveiy. l

:-B.JA.1iisoriminator oircnitmc-omprisingaa pairrofOPpOSedIeC-tierSfeaeh having anr output loadv impedancegfrsaidrrectiersproviding ,fanV outputgpotential "which is fthe diierence :between .the'.po-

tentials 1 nreated across wtheir respective.. load l:im-

pedances, #a source .of "angie :modulated carrier energy,'means for:feedingfenergy from said source to said :rectiers rthroughdifferent...channelswre spootivelysandV vacuumntube; amplifier circuits:in said :schannelsfresponsiverto direct. current-,ipotentiarls across:said impedances "forsde'creasing the eiciencyrofztransf erofeenergyioeonefof saidrrectiersf through: itsnespective'channel.relatively .to .the fefoienoytof Ytransformer fenergy itof said otherreotierthroughsthe viotnerrof 'said channels, or viceversa, :dependingupon change. of the carrier frequency in Vone direction l:or @the :otherrespectively..

:9.111 af'system oftheztype described, apairff opposedrectiers.eachhavingfanoutputioadimpedante, said :.rectiers :providing.an zoutput potential which is the :difference between thepoten-tialscreated across theirsrespectivezload'impedanees,y a `source of efanglefrnodulated fcari'ier energy;l means fon feeding` energy from:saidesource to :said frectiers through @different channelsrespectively, :at :least :one rvacuum tube.' intxeach fof said mannelsiorspassing; onfthe alternating :current energy impressed ffupontheinputs :of said tubes; and rmeans for .'.corrtroilingA the transfer: ofsignal'energy inea `tute .irrione of seitlichannels in accordance'withfdirectacurrent apotentiaisiderived romthenutputsnf saidi'rectierstodecrease'the -efiiciency` rof-f'transfer ofizenergy'to ione-ofsaidxrectiers through; its respect-Ive 'channel relatively tothenefficiency of "transfer 'offenergy-tonsaid tether rectierfthrougnthe :other of. said channels,

lil-A :frequency lmeciuiai'fion detector circuit comprising 'a pail-vof:opposedzeetierseach have ing an" output load impedance,- saidrectierszproviding an output potential which is the difference betweenthe potentials :created -aroross their :resposti-ve dead l impedances, al,source `orl frequency modulated :carrier :'eneogy, means .for feedingTenergyifrom said'gsourcefto-:said' rectiers" 16 dierent: :channels."respectiveiy, resonant circuit meansr--m said .channes vforkintreasing signal energy transierr tonne of. said: rectiersrelativelytoithe father: uponfchange 'of signalgfrequeneyf in one .direction. -:orthe other zrespeetiveiy,V and therminnicwaeuumrtube means responsiveztodirectfcurrentqaotentiis across-:sadimpedancesfor decreasing 'theefciency of :transf-er `rof :energy toitheriof saidrectiers when fineVcarrierzfrequency fof-the :received: 'signa-1 movesv in a lAvchiraction, .u suchtas otherwise to vcause increaseioffenergytransfenztosaidrectier. Y

11. In combination withfazpair of'resonant'circuits shaving :respective."-ampiitudee-requency responsercharacteristics' with .differentspeakzire qnencies,.=a separatepair offoutputfterniinaisifor eachlresonant foir'euit,` a resonant :signal finput eircuit'tuned .tofaffreouency vlocated equidistant- 1y` iremfsaid peak frequencies, a pairoft'parallel signal transmission channels l-having a :common inputconnectionto said input circuit, eachchanneifbeingv connected lto arespective one ofi thepair ofresonant oir-cuits7 and means fiorregulating the reiative signal transmission eiciencies 1 of saidparalielchannels.

12,-Iny combination with -azpair of resonant'circui-ts "having:respective a-mplitude-irequenoy 4response I:tirare-eteiisftics :with"different 1 peak frrequencies, a pair of outputterminals'ioreaohresonanteircuit, :alsignal inputvcircuiti'tuned to afrequency located 'between saidz :peak frequencies, saidresonant-circuitsfnaving frequency response curves l-v'vhieh cross eachother -substantialiy at said -input 'circui-t' 1 frequency', 'a pan* :ofparallel signaiftransnnsson channels having a 'common input' connectiontosai'd input' Circuit; each channel'lbeingfconnecte'dto a 'respectiveone kof the pair of fresca-ant :circuits and means Vforzautomatically-regulating the reiativelsi-gnal'-transmission-eileiemziesofr` said f parallel charmels to v'providesubstantially 'equal signal voltages across said resonant .circuits;

13. In combination Withfafpair offresonantfcireu-its'ftuned to`Vdifferent resonant frequencies a pair of output terminals 1'fereachresonant f oircuit, `af-'signal y input 'circuit 4tuned tofa frequencylocated equidistantlfy `from said resonant '-frequenc'ie's, ra fpair ofparallel signal 'transmission channels-:having a common input connectionf :to said input:crouihseachrchannel being connected to a respective oneof the pair of resonant oircdits, #and means, responsive toy .adifference hetween the mean frequency ofi Vapplied frequencyvariable:signals and said inputxoircuit frequency, 'forx regulatingfthe:relative .signal transmission lefiicienciesof said parallei' channels.

14. In a systerntorhreceiving frequency modulated :carrier lWaves., a'frequency discriminator net work. for derivingffrom received .wavesaipair ofzrnodulated V'Wave voltages i in phase quadrature relation'in'response: to the mean* frequency Otsaid received 'waves being equal4to apredetermined center-balancerfrequencma pair of sepai-'atetrans-4mission channelsifori the vrespective vo1tages,;.sep arate A:rectiiersvrespectively -connected to .said .channeis,;means.:combining :therectified irvoltage outputs fof said 1 'rectiers in .polarity:.opposition, .amd `means, "responsive yto Ia :departure-tof said meantreqnency from fsaid center :balance :irequencmior regulating therelative'zwave transmission :efcences :of zsaid in a lsense ttocompensate for said departure.

:Inraiststemrforreceiving frequency modulated carrier .wvaves, safrequency discriminator network forxfderivingnfrom receivedfwavesa pairquency of ,saidreceived Waves being equal to av predetermined centerbalance frequency, a pair ofseparate amplifier channels for therespective voltages, said channels being each tuned fto saidcenterbalance frequency, separate rectifiersre` spectivelyconnected tosaid channels, means comu, bining'the rectified voltage outputs of saidrectifiers in polarity opposition, and means connected to .saidcombining means, responsive `to a departure of said mean frequency fromsaidcenter balance frequency, for regulating the relative"I gains ofsaid ampliiierchannels in asense to r'ap-vy ply the Wave'voltagessubstantially equallytosaid rectiiiers.

16. In 'a frequency detector of Vthe type com-V circuits each.

prising apair of decoupled resonant feeding into a respective rectifierand said resonant circuits having amplitude-frequency characteristicswhich overlap a predetermined terbalance frequency between the peakfre-4 quencies of the characteristics; theimprVement which comprises atleast two parallel amplifiers having a-cornmon input connectionadapted'to be connected toa source of frequency modulatedsig-Y nal"energy, separate connections from each ampliii'er to a respective one ofthe resonant circuits,

means responsive to the differential outputfvolt?` ag of, the rectiersfor varying the gainof one-- of said tvvoamplifiers in one sense, andmeans responsive to a change in the gain of "saidone amplifier forvarying the gain of the second am-'U plier in the opposite sense.

17. In combination, a pair of decoupled resonant circuits havingrespective amplitude-frequency e response characteristics with differentpeak frequencies, a pair of output terminals for each resonant circuit,a resonant signal input circuit tuned to a frequency locatedequidistantly from said peak frequencies; and the improvementcomprising' a pair 'of parallel signal transmission channels having acommon `inputV vconnection Utdi said input' circuit, each channel beingconnected' to a respective one of the pair of resonant circuits, andmeans for regulating the relative signal transmission efliciencies ofsaid parallel channels in opposite senses.

18. In a system of type comprising a pair of resonant circuits havingrespective amplitude-frequency response characteristics with differentpeak frequencies, a pair of output terminals for each resonant circuit,a resonant signal input circuit tuned to a frequency locatedequidistantly from said peak frequencies, said characteristics beingoverlapped and having a cross-over frequency substantially equal to saidequdistant frequency; the improvement comprising a pair of parallelsignal transmission channels having a common input connection to saidinput circuit, each channel being connected to a respective one of thepair of resonant circuits, and means for regulating the relative signaltransmission eiciencies of said parallel channels in opposite senses andto an extent such as to maintain signal voltages at said pair ofresonant circuits substantially equal.

19. In a system of the type comprising a pair of resonant circuitshaving respective amplitudefrequency response characteristics withdifferent peak frequencies, a separate rectifier for each resonantcircuit, a signal input circuit tuned to a frequency locatedequidistantly from said peak frequencies; the improvement comprising apair of parallel signal amplifiers having a common input connection tosaid input circuit, each am- @meer pliiier beingconnected tov arespective one of the pair of resonant circuits, and means, responsivevto a differential voltage output of the rectiers u caused by'adifference .between the mean fre-- quencyof`- applied frequency-variablesignals and, saidinput ci uit frequenfor,regulatingthev l reia egai soflaidparallel.ampliiiera,r

l295m `'asfystemfofr receiving angle" modulated carrierWavesmmeans'f',deriving from received', wavesia pair of modulated'wavevoltages in phase quadrature relation-in response to the mean Yfreiquency-'of saidlreceved Waves beingequal-to a. predeter rlfledffc'eJ*balance frequency, a pair of`separate amp irs for the respectivevvoltages, separatefcnectiiiers respectively connected l to saidainpliers, means combining the rectified voltageV outputs of Vsaidrectiiiers in polarity opposition, a` and means, responsiveto adeparture of said mean frequency from said-fbaiance frequency, forregulatine' :ihefeeie-S.. einem.;emeliierainregeeeite i Seil esteeefiieeeeeteiereeie-departure.

2 systerniel receiving frequency. mesuf er Wares, aireeuenev..diesrimiceter ved avesenerfi 'effvelteeee'laquadrature f. Seie-themeenf;.quency-0f sa, eine lequel. te e credete, mi e frecuentas Pair Q f'parele-feseeeive velteeeereeparei con ected to `'said ampliiiers,

il' relie, Ouipilts ef. 1nee1e. we 'iioaandneeris l said,A o bid'gglmean 'responsivetoq gli ,eiecto@f ltheypercfzmle, f pair off,resonant circuits each feeding ,speetive diede et ii1ier,: eai :1: 1 Ieveeampliiuei .ireineneinCharactert. eiiee, uhieh, Pueden, atlarzrerilecimed, beteeee nalefliegeences'1.l ofv the c aracteristic theimprovement which comprises a source of frequency modulated signalenergy, at least two parallel amplifiers having a common inputconnection to said source, a separate output connection from eachamplifier to a respective one of the resonant circuits, means varyingthe gain of one of said two amplifiers in one sense in response to saidone amplifier feeding a predominant signal voltage to its respectiveresonant circuit, and means responsive to a change in the gain of saidone ampliiier for varying the second amplifier in the opposite sensethereby to substantially equalize the application of signal voltages tosaid rectiers.

23. In combination with a frequency discriminator comprising a pair ofresonant circuits having respective amplitude-frequency responsecharacteristics of different peak frequencies, a separate detector foreach resonant circuit, a resonant signal input circuit tuned to afrequency located between said peak frequencies, a pair of parallelsignal amplifiers having a common input connection to said inputcircuit, each amplifier being connected to a respective -one of the pairof resonant circuits, and bias control means responsive to the outputsignal of said detectors for automatically controlling the gain of saidparallel amplifiers so as to feed substantially equal signal voltages tothe detectors,

24. In combination with a frequency discriminator comprising a pair -ofresonant circuits having respective amplitude-frequency response 25. Incombination with a pair of oppOsed clev tectors of angle modulatedcarrier waves, saidv detectors" having respective vfrequency 'Y responsecurves which cross each other at a predetermined cross-over frequency,parallel amplier channels feeding :modulated carrier'wave voltages to'said detectors, Vand a bias control connection for said amplifierchannels responsive to the detector outi puifvoltage controlling therelative gainof said channels thereby to adjustsaid response curvessufliciently to change said'crossQoverfrequency.

26.- In combination, a pair of resonant circuits havingv respectiveamplitude-frequency response curvesv with` dilerent peak frequencies,said re'- sponse curves-crossing each other'at a desired frequency, apair of output terminals for each resonant circuit, the improvementcomprising a pair of parallel signal transmission channels having'acommon signal input connection, each channel being connected to arespective one of the' pair of resonant circuits, and bias controlmeans'responsive to the voltage developed across said pair of resonantcircuits regulating therelative response curves of said parallelchannels to adjust-'said cross-over frequency.

27. In a system for receiving frequency'moduf latedcarrier waves, afrequency discriminator y network for` deriving from received wavesapaii" off modulated wave voltages of equal value in response Yto themean frequency `ol? said received waves vbeing equal to apredeterminedcenter balance frequency, a pair of parallel amplifiers for antigne the"respective voltages, separate "rectiers/ indi-f vidually connected tosaid amplifiers, 'meansco'm'e' bining the rectified voltage outputs ofsaid rectif," fiers in polarity opposition, anda loiasl icfontr'ol-f4connection for each ofsaid ampliiiers'respons'ive'- to a differentialoutputlvoltage from saiddef' tectors regulating thev relative `gains ofsaid fampliers in opposite senses to compensate for departure of saidmean frequency from said bal` ance frequency. l ,l I y l,

28. In detector of the .typevc'omprising al', ir' l of resonantcircuitseach feeding into a respec v'e rectifier, said resonant circuitsAhaving amplitud frequency characteristics whichroverlap 'at apfel-fdetermined center balance frequency betvve'enth v` peak frequencies ofthe chaulracteristics;l the provement which comprises a source offrequency u modulated signal energy, at least two parallel"j ampliershaving a common input ,con'nectionI toy ,l said source, a separateoutput connectionironr',l each'ampliier to a respective one oftheresonantr circuits, means responsive to the dierntialoutl."

put of the rectiers for varying the ygaiiiofp'ne l of said twoamplifiers in one sense in resr'io'nse,` o

l said one amplifier feeding a predominant" signal'-2 voltage to itsrespective resonant circuit, fand means in the space current path lof'saidone amplirier responsive'to a change in thevgainof f said oneampliiier for varying thejseco'nd a' pliiier in the oppositev sensetherebyto substan'-v tially equalize the applicationv of signal voltagesto said rectiiiers.

WENDELL L.l CARLSON;

l REFERENCES CITED The following references are of recordin 'A i'ileofthis patent: l UNITED STATES PATENTS"v Number Name ,Datemf l 2,264,151Reid Nov1'25, 1941. 2,351,193 Crosby June113,1944r 2,356,201

